When using an air doctor, the coating mix applied to the web is smoothed by directing a high-velocity air jet via a slot-orifice nozzle of the air doctor toward the web. This air knife removes the excess coat from the web surface in the form of a coat mist which is collected into a specially designed blow-off hood and recycled back to the coating mix pan. With the help of the air doctor, a smooth coat is attained and the profile of the coated paper or paperboard web follows the contour of the base web. The opacifying power of the applied coat is good. However, this method is not suitable for applying high-solids coats.
The greatest drawback of air doctoring is its inherently weak blow-off capability of removing the excess coat, which capability is further impaired at higher web speed. Consequently, air doctoring must employ coating furnishes of low viscosity and solids content, and even so the usable web speed remains less than 500 m/min in even the fastest machines. For these reasons, air doctoring is used almost exclusively in board coating where good opacifying power is imperative and high web speeds are not as critical as in papermaking in general. If the viscosity or solids content of the coating mix is increased, the air doctor loses its ability to blow off the excess coat and, therefore, the finished coat weight becomes excessively heavy. Accordingly, the requirements set for air doctor coating are that the applied coat should be as smooth as possible and the weight of the applied coat should closely approximate the desired finished coat weight.
U.S. Pat. No. 3,235,401 discloses an air doctor apparatus in which the web to be coated is directed, first to a metering roll of the applicator apparatus via a guide roll. The metering roll is placed in the coating mix pan so that the lower part of the roll is immersed in the pan, while the web runs over the upper part of the roll. The metering roll lifts an excess amount of the mix from the pan to the web which then passes over a rotating predoctoring rod that removes a portion of the excess coat from the web. The purpose of the predoctoring rod is to smooth the coat and remove so much of the excess coat that the air knife can then doctor the coat to the desired finished coat weight. After the predoctoring rod, the web travels onto a backing roll having a closely-disposed air knife so as to blow a narrow-slitted air jet in the reverse direction to the web travel and to thus doctor the coat to its finished weight.
Several variants of the above-described type of apparatus are known in the art, and they constitute the basic construction or use of air doctors. A drawback of these doctor apparatus is the rapid decrease of their doctoring performance in terms of coat quality and smoothness at higher web speeds.
Patent publication WO 91/17309 discloses an apparatus which is further developed from that described above in that the coat quality and maximum usable web speed during coating have been improved. The apparatus described in WO 91/17309 is otherwise similar to the apparatus described in U.S. Pat. No. 3,235,401, except that the applicator roll is complemented by a doctoring bar which performs both smoothing and metering of the coat transferred from the coating mix pan to the web. In this fashion, the coat applied to the web attains better smoothness and the coat weight is more accurately reduced to the desired finished coat weight. Such an arrangement has the advantage that the air knife need not remove a great amount of excess coat and the coat will have improved smoothness since the initially applied coat is already relatively smooth. Bar smoothing of the coat applied to the web also improves the quality of the end product and permits a higher web speed owing to the reduced blow-off duty imposed on the air knife. In addition, bar smoothing obviates the need to use a rotating predoctoring/metering roll.
Though the above-described apparatus is capable of overcoming certain drawbacks of air doctor techniques, there remain several disadvantages mostly related to the applicator roll method. When running at a high web speed, the applicator roll causes vigorous splashing of the coating mix which then finds its way all over the machinery, including the web and the surroundings. As the rotational speed of the applicator roll is greatly increased at higher web speeds, splashing becomes particularly problematic at the highest web speeds. When using an applicator roll, uncoated spots will remain on the web. Further, the web tension profile has a significant effect on the thickness of the applied coat, and since the air doctor is incapable of smoothing away large variations in coat weights, changes in the web tension profile are directly evidenced by quality defects. Moreover, the roll applicator is characterized by a type of inherent quality defect, namely, the orange peel pattern caused by the splitting of the coat film at the outgoing side of the contact point between the web and the applicator roll; this orange peel pattern cannot be effectively removed by means of air doctoring, particularly if the web speed is high.
A roll applicator cannot be used for applying low coat weights on the web since it results in mottling of the web with uncoated spots. Furthermore, the control of the cross-machine profile of the applied coat becomes rather impossible.